Sources & formulas

Every calculator on GutterCalcs rests on established drainage geometry plus stable gutter conventions. Unlike topics with a time dependency, “verification” here is mathematical: each formula is tested against known values and is therefore correct for good. Here are the technical foundations by area.

Identities & geometry

  • Linear feet: the sum of the eave run lengths (gutters go on the eaves, not the gable rakes).
  • Effective roof area: footprint × a roof-pitch multiplier = √(1+(rise/run)²).
  • Gutter size: the smallest size whose max drainage area ≥ the effective roof area, from the labeled rainfall × gutter-size table; max area(intensity) = max area at 6 in/hr × 6 ÷ intensity.
  • Downspouts: ceil(effective area ÷ max area per downspout) or ceil(linear feet ÷ spacing); use the larger.
  • Quantities: sections = ceil(linear feet ÷ 10-ft stock); hangers = ceil(linear feet ÷ hanger spacing); end caps = 2 × runs; elbows = 3 × downspouts + 2 per offset.
  • Rainwater: gallons = footprint × rainfall inches × 0.6234 × runoff (1 inch over 1 ft² = 144 in³ = 0.6234 US gallons).

Convention typicals (labeled)

  • Roof-pitch multipliers: 0/12 = 1.000, 4/12 ≈ 1.054, 6/12 ≈ 1.118, 8/12 ≈ 1.202, 12/12 ≈ 1.414 — the geometric identity.
  • US rainfall intensity: ~2–9 in/hr (5-minute / 100-year) by region — labeled, confirm your local rainfall.
  • Max roof drainage area per gutter size (SMACNA-style): e.g. at 6 in/hr, 5" K-style ~2,500 ft², 6" K-style ~3,840 ft², 5" half-round ~1,920 ft² — labeled, confirm local code.
  • Downspout capacity: 2×3 ~600 ft², 3×4 ~1,200 ft², 3" round ~700 ft², 4" round ~1,250 ft²; 1 per 30–40 ft — labeled.
  • Fastening & stock: hanger spacing ~24–36 in, 10-ft stock length, slope 0.25–0.5 in / 10 ft, runoff ~0.90–0.95, waste ~5–10% — labeled.

Cost tools

  • Every cost tool uses the prices you enter ($/linear-foot, $/downspout, $/elbow, labor $, removal $, $/lf guard, cleaning $/lf) — no material or labor price is stored, so the site needs no maintenance. Cost bands are a labeled sanity guide.
  • Cost: total = (quantity × your $/unit + downspouts + labor + add-ons(guards, fascia repair, removal, two-story access) − discount) ×(1 + contingency).
  • Quote check: derived $/ft = quote ÷ linear feet, compared to the labeled band for the material — a sanity flag, not a bid.

Material and labor prices, product data-sheet values, code and local rules vary by place and change over time — always measure your eave runs and confirm capacity, spacing and coverage against the exact product, order a little extra (~5–10%) for corners, waste and slope, follow local code and the manufacturer’s data, defer structural roof-load, roofing and foundation/yard drainage to a certified professional, and get itemized written quotes from a licensed, insured gutter contractor before you commit.